Optical disc

ABSTRACT

An optical disc has a support base and at least one information-retrieval layer formed over the support base. The information-retrieval layer is transparent and thinner than the support base. The information-retrieval layer exhibits variation in birefringence within ±20 nmpp (nano meter peak to peak) under double-pass measurements during one rotation of the optical disc.

This is a divisional application of Ser. No. 10/973390 filed Oct. 27,2004 now U.S. Pat. No. 7,011,878, which in turn is a divisional of Ser.No. 10/243,827 filed Sep. 16, 2002, now U.S. Pat. No. 6,815,032.

BACKGROUND OF THE INVENTION

The present invention relates to an optical disc having aninformation-retrieval layer for enhanced high signal quality inrecording and reproduction.

Optical discs are a storage medium having a transparent plastic supportbase on which information signals are recorded as minute uneven pittrains or grooves. The information signals are retrieved by emission oflaser beams from the surface opposite to the information-recordedsurface while intensity of the reflected laser beams is varying inaccordance with the recorded information.

This type of optical disc has been widely used, one representative beingcompact discs (CD) usable at 780 nm-wavelength of recording andretrieving laser beams and 0.45-aperture number of optical-pickupobjective lens.

The types of compact disc ranges from read-only type discs (ROM) forinformation retrieval only, recording type discs (CD-R) to whichrecording is allowed only once to recording/retrieval type discs (CD-RW)to which recording, retrieval and erasure are allowed several times.Another type is magneto-optical discs having shape similar to thesecompact discs.

Thanks to reasonable cost for short-wave laser devices and advanceddisc-manufacturing technology, the most popular disc is a digitalversatile disc (DVD) usable at 635 nm-wavelength of recording andretrieving laser beams and 0.6-aperture number of optical pickupobjective lens. High disc density has been achieved thanks to shortlaser wavelength and high objective-lens aperture number.

Moreover, expected as the next-generation discs are optical discs moredense than DVD, usable at 400 nm-wavelength (or shorter) of recordingand retrieving laser beams and 0.7-aperture number (or higher) ofoptical-pickup objective lens.

The thickness of transparent support base via which information signalsare retrieved for optical discs, such as CD, is 1.2 mm usable at 780nm-wavelength of recording and retrieving laser beams. Formed on thetransparent support base is an information-recorded layer such as pittrains and grooves. The information on the information-recorded layer isretrieved from the rear surface. The transparent support base having theinformation-recorded layer is formed as a transparent retrieval layer.

Optical discs, such as DVD, more dense than CD, have a 0.6 mm-thicktransparent information-retrieval support base usable at 635nm-wavelength of recording and retrieving laser beams. Formed on thistransparent support base is an information-recorded layer such as pittrains and grooves. The information on the information-recorded layer isretrieved from the rear surface. The transparent support base having theinformation-recorded layer is formed as a transparent retrieval layer,like CD. This type of optical discs, however, have a 0.6 mm-disc, calleddummy disc for high disc strength.

The next-generation optical discs, usable at 400 nm-wavelength ofrecording and retrieving laser beams, have an information-recorded layersuch as pit trains and grooves formed on a support base having thicknessin the range from 1.1 to 1.2 mm. Stuck on the information-recorded layeris a sheet having thickness 0.2 mm or less for recording and retrievalvia this sheet surface. One type of next-generation optical disc hassuch information-recorded layer formed on a transparent retrieval layer.

As discussed, the thickness of optical disc has become thinner as thewavelength of recording and retrieving laser beams has become shorter.In addition, increase in objective-lens aperture number in opticalpickup contributes to further high density.

The next-generation discs usable at 400 nm-wavelength of recording andretrieving laser beams and 0.7 (or higher)-aperture number ofoptical-pickup objective lens usually have thickness of 0.2 mm or lessfor the information-retrieval layer from the information-recordedsurface on which pits or grooves have been formed as information signalsto the information-retrieval surface.

Such a thin information-retrieval layer prevents laser beams fromdiverging during recording and retrieving due to increase in opticalaberration such as spherical aberration caused by increase in lensaperture number and coma aberration caused when the disc is bent.

The thinner the information-retrieval layer, however, the more difficultin forming an information-recorded layer on an information-retrievallayer by injection molding like DVD when the support base is 0.6mm-thick. This is because molding resin will not be filled enough in amolding die by injection molding when a support base is too thin againstits diameter, resulting in incomplete formation of information-recordedlayer.

To solve such a problem, these next-generation optical discs aremanufactured by forming an information-recorded layer on a support basehaving a thickness of 1.1 mm, for example, by injection molding,followed by forming an information-retrieval layer with an aluminumreflecting film on the information-recorded layer by sputtering, abonding layer formed on the reflecting film and further a transparentplastic disc sheet having a thickness of 0.1 mm, for example, on thebonding layer.

Evaluation tests revealed, however, that these next-generation opticaldiscs having a 0.1 mm-thick transparent plastic disc as theinformation-retrieval layer suffer from large variation in reproducedwaveforms per one disc rotation. It is further revealed that one cycleof variation in reproduced waveforms mostly has two-cycle components.

An automatic gain controller suppresses such variation a little bit, butstill not enough against asymmetry and degraded frequencycharacteristics of the varied output waveforms, thus providing noconstant output.

Several sample discs suffered from peak-to-peak variation in reproducedwaveforms, some reproducing output almost half of the minimum level.

The variation in reproduced output peculiar to the next generationoptical discs having such thin information-retrieval layer causeincrease in output errors, an inevitable halt to normal retrievaloperation.

SUMMARY OF THE INVENTION

A purpose of the present invention is to provide an optical discexhibiting preferable reproduction characteristics while suppressingvariation in reproduced output.

The present invention provides an optical disc includes: a support base;and at least one information-retrieval layer formed over the supportbase, the information-retrieval layer being transparent, thinner thanthe support base and exhibiting variation in birefringence within ±20nmpp (nano meter peak to peak) under double-pass measurements during onedisc rotation.

Moreover, the present invention provides an optical disc includes: asupport base; and a transparent sheet acting as an information-retrievallayer, stuck over the support base, the sheet being thinner than thesupport base, the sheet exhibiting variation in birefringence within ±20nmpp (nano meter peak to peak) during one rotation under double-passmeasurements.

Furthermore, the present invention provides an optical disc includes: asupport base; and a transparent sheet acting as an information-retrievallayer, stuck over the support base, the sheet being thinner than thesupport base, a draw ratio for the sheet being adjusted so that thesheet exhibits variation in birefringence within ±20 nmpp (nano meterpeak to peak) during one rotation.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional view illustrating an embodiment of an optical discaccording to the present invention;

FIG. 2 illustrates a method of manufacturing the optical disc shown inFIG. 1;

FIG. 3 is a graph indicating the results of double-pass measurements ofbirefringence on sheets to be used as an information-retrieval layer;

FIG. 4 is a graph indicating signal output reproduced from the opticaldiscs according to the present invention; and

FIGS. 5A to 5D are sectional views illustrating modifications to theembodiment of optical disc according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments according to the present invention will bedisclosed with reference to the attached drawings.

As shown in FIG. 1, an optical disc D1 according to the presentinvention has a disc-like support base 2 having a specific thickness.Formed on the surface of the support base 2 is an information-recordedlayer 4 of pit trains or grooves of fine concavity and convexitycarrying information signals, with a very thin reflective thin film (notshown) formed on the layer 4. Bonded on the thin reflective thin filmvia a bonding layer 6 is an information-retrieval layer 8 of a very thintransparent sheet.

The support base 2 may be made of polycarbonate resin. The reflectivethin film may be of an aluminum film. The bonding layer 6 may be made ofultraviolet-hardened resin. The information-retrieval layer 8 may be ofa plastic (resin) sheet of polycarbonate. In addition, the plastic(resin) sheet may be covered with a hard coat (not shown) againstdamages for a double-layered information-retrieval layer.

One of the features of the optical disc D1 according to the presentinvention is that the information-retrieval layer 8 made of such a verythin transparent sheet is thinner than the support base 2.

Another feature of the optical disc D1 according to the presentinvention is that variation in birefringence on theinformation-retrieval layer 8 during one rotation of the disc D1 is setwithin ±20 nmpp (nano meter peak to peak) among components ofbirefringence under double-pass measurements.

In a method of manufacturing the optical disc D1, as shown in FIG. 2, adisc-like sheet almost the same size as the disc-like support base 2 iscut out from a roll of a plastic sheet 10 having the characteristics ofbirefringence within ±20 nmpp during one rotation under double-passmeasurements. This cut-out sheet is employed as theinformation-retrieval layer 8.

A bonding layer 12 of ultraviolet-hardened resin is placed on theinformation-recorded layer 4 that has already been formed on the supportbase 2. The disc-sheet information-retrieval layer 8 is placed on thebonding layer 12. The support base 2 is rotated by a spinner to spreadthe ultraviolet-hardened resin over the layers. The resin is hardened bya UV (UltraViolet) light to join the layers, thus finishing the opticaldisc D1.

It is achieved that variation in reproduced output from the optical discD1 is drastically suppressed because of reduction in birefringencevariation during one disc rotation.

The thickness of the sheet 10 to be used for the information-retrievallayer 8 is preferably 0.2 mm or less. The minimum thickness for thesheet 10 is, however, about 50 μm at present due to difficulty inproduction.

The birefringence variation during one disc rotation is preferablywithin ±10 nmpp (nano meter peak to peak) under double-passmeasurements.

Disclosed next are several methods of producing a sheet to be used forthe information-retrieval layer 8.

A transparent sheet having a thickness of 0.2 mm or less is usuallycalled a just sheet available in the market.

In general, melted plastic is kneaded and passed through a pair ofrollers having a specific gap, thus producing a sheet of a specificthickness. The sheet is processed into a final thickness and flatness bya calender roll and a drawing machine. Drawing may be performed in thedirection of sheet length or in two directions of sheet length andwidth. The more drawn, the higher the elasticity for some types of sheetmaterial.

Or, a sheet a little bit thicker than a final product may be produced byextrusion molding. This sheet is drawn by a calender roll for desiredthickness and flatness. Moreover, resin is dissolved in a solvent andthen spread into a sheet while the solvent is evaporating with almost nodrawing operations.

The inventors of the present invention selected a sheet exhibitingvariation in birefringence within ±20 nmpp (nano meter peak to peak)during one rotation under double-pass measurements, among several sheetsproduced as explained above.

The selected sheet was cut into the size of a disc support base. Atransparent information-retrieval layer was formed on aninformation-recorded layer via a transparent bonding layer over thesupport base to finish several sample optical discs.

The sample optical discs were tested for evaluation of reproducedoutput. All samples produced in several ways exhibited variation inreproduced output synchronized with one disc rotation. Many of themexhibited two-cycle variation in output.

These sample discs were set on optical-disc birefringence measuringequipment offered by Adomond Science Co. for birefringence on theinformation-retrieval layer. The measurements showed two-cycle variationin birefringence on the information-retrieval layer per disc rotationwith the less variation in birefringence, the less variation inreproduced output per rotation.

It is then revealed that variation in reproduced output per rotation iscaused by variation in birefringence on the information-retrieval layer.

Also found through the measurements is that the lower the draw ratio,the less variation in birefringence occurs on the information-retrievallayer. It is thus revealed that variation in birefringence has arelation with draw ratio in sheet production. The cause seems to be aheavy stress applied to the sheets in the direction of drawing whilebeing drawn. Such a heavy stress gives stress strain to the sheets tocause orientation of plastic molecules in the sheets, thus the moreorientation, the more birefringence occuring.

The measurements taught that variation in birefringence during onerotation can be suppressed to a certain degree or less under drawingcontrol in sheet production.

Under the measurements, the optical disc according to the presentinvention can be manufactured by using a disc transparent sheet thatexhibits variation in birefringence at a certain level, for example,within ±20 nmpp (nano meter peak to peak) during one rotation, as theinformation-retrieval layer.

Materials of the plastic sheet to be used as the information-retrievallayer according to the present invention are transparent and preferablyexhibit high thermal deformation temperature but low birefringence whenformed into a sheet. Such materials are, for example, polycarbonate,polystyrene, amorphous polyolefine and acetate. Among them,polycarbonate is the best choice for its reasonable price and highyields in sheet production. Nevertheless, polycarbonate resin exhibitshigh birefringence (optical elasticity) originated to resin.

Drawing is, therefore, controlled so that orientation of polycarbonatemolecules will be suppressed for low birefringence (optical elasticity)to provide sheets suitable for the optical discs according to thepresent invention, even from resin exhibiting high optical elasticity.

Disclosed next are several embodiments of optical disc according to thepresent invention and sample optical disc for comparison.

Embodiment 1

An optical-disc molding die was attached on an injection moldingmachine. A stamper for information formed had already been set in themolding die. The stamper was made of nickel, formed on which was abit-train information track having a pit depth of 50 nm, the shortestpit length of 0.19 μm and a track pitch of 0.35 μm.

A support base 2 having a diameter of 120 mm, a bore diameter of 15 mmand a thickness of 1.2 mm was made from the optical-disc molding die.

The support base 2 was set in a sputtering system. A reflective thinfilm of aluminum having a thickness of 50 nm was formed on aninformation-recorded layer 4 of the support base 2.

The support base 2 was set on a spinner as the aluminum reflective thinfilm was on top of the base. An ultraviolet-hardened resin was droppedonto the reflective thin film to form a bonding layer 6.

A sheet 10 made of polycarbonate having a thickness of 100 μm,exhibiting variation in birefringence within ±20 nmpp (nanometer peak topeak) during one rotation under double-pass measurements was cut into adoughnut-like information-retrieval layer 8 having a major diameter of119 mm and a minor diameter of 38 mm. The polycarbonate sheet 10exhibited a low draw ratio.

The information-retrieval layer 8 was placed on the bonding layer 6 androtated by the spinner to spread the ultraviolet-hardened resin over thereflective thin film to have a specific resin thickness. The sheetinformation-retrieval layer 8 was exposed to a UV light, so that thesheet was stuck on the support base 2, thus finishing an optical disc Daaccording to the first embodiment.

The optical disc Da was set on the optical-disc birefringence measuringequipment offered by Adomond Science Co. Birefringence was measured withlaser beams having a wavelength of 780 nm incident at 80 to 90 degrees(almost vertical) while the optical disc Da was rotating. Themeasurements showed two-cycle variation in birefringence during onerotation of the optical disc Da, which was ±20 nmpp (nano meter peak topeak) under double-pass measurements, as shown in FIG. 3.

Moreover, the optical disc Da was set on a next-generation optical-discplayer for an evaluation test at a wavelength of 400 nm for theretrieving laser beams and an aperture number of 0.85 for anoptical-pickup objective lens.

The evaluation test showed two-cycle variation in peak-to-peakreproduced output during one rotation of the optical disc Da, as shownin FIG. 4. In detail, the variation in reproduced output was 15%obtained by dividing the difference between the maximum output V_(H) andthe minimum output V_(L) by the maximum output V_(H), or(V_(H)−V_(L))/V_(H).

Decreased with the variation in reproduced output, as shown in FIG. 4,were asymmetrical variation in reproduced output and a ratio of theshortest-length bit output to the longest-length bit output. Moreover,almost no errors occurred in decreased signal output. Overall evaluationof the optical disc Da according to the first embodiment resulted inhigh signal quality and stable tracking.

Embodiment 2

An optical disc Db according to the second embodiment was manufacturedthrough the same procedures as the first embodiment, using apolycarbonate sheet having a thickness of 100 μm but exhibiting a drawratio lower than that for the sheet used in the first embodiment.

The polycarbonate sheet used in the second embodiment exhibited ±10 nmpp(nano meter peak to peak) two-cycle birefringence during one discrotation under measurements with the same birefringence measuringequipment as the first embodiment.

The evaluation test with the same next-generation optical-disc player asthe first embodiment showed 10% two-cycle variation in reproduced outputduring one rotation of the optical disc Db.

Decreased with the variation in reproduced output were asymmetricalvariation in reproduced output and a ratio of the shortest-length bitoutput to the longest-length bit output. Moreover, almost no errorsoccurred in decreased signal output. Overall evaluation of the opticaldisc Db according to the second embodiment resulted in high signalquality and stable tracking.

Embodiment 3

An optical disc Dc according to the third embodiment was manufacturedthrough the same procedures as the first embodiment, using apolycarbonate sheet having a thickness of 100 μm but exhibiting a drawratio lower than that for the sheet used in the second embodiment.

The polycarbonate sheet used in the third embodiment exhibited ±5 nmpp(nano meter peak to peak) two-cycle birefringence during one discrotation under measurements with the same birefringence measuringequipment as the first embodiment.

The evaluation test with the same next-generation optical-disc player asthe first embodiment showed 5% two-cycle variation in reproduced outputduring one rotation of the optical disc Dc.

Decreased with the variation in reproduced output were asymmetricalvariation in reproduced output and a ratio of the shortest-length bitoutput to the longest-length bit output. Moreover, almost no errorsoccurred in decreased signal output. Overall evaluation of the opticaldisc Dc according to the third embodiment resulted in high signalquality and stable tracking.

[Sample 1]

An sample optical disc Dsa was manufactured through the same proceduresas the first embodiment, but using a polycarbonate sheet having athickness of 100 μm under drawing in two directions.

The polycarbonate sheet used for this sample exhibited ±45 nmpp (nanometer peak to peak) two-cycle birefringence during one disc rotationunder measurements with the same birefringence measuring equipment asthe first embodiment.

The evaluation test with the same next-generation optical-disc player asthe first embodiment showed 25% two-cycle variation in reproduced outputduring one rotation of the sample optical disc Dsa, which was largerthan the first to the third embodiments.

Increased with the variation in reproduced output were asymmetricalvariation in reproduced output and a ratio of the shortest-length bitoutput to the longest-length bit output. Moreover, several errorsoccurred in decreased signal output. Overall evaluation of the sampleoptical disc Dsa resulted in low signal quality and unstable tracking.

Embodiment 4

A phase-change-optical-disc molding die was attached on an injectionmolding machine. A stamper for information formed had already been setin the molding die. The stamper was made of nickel, formed on which wasa spiral-groove information track having a groove depth of 27 nm and atrack pitch of 0.32 μm.

A support base 2 having a diameter of 120 mm, a bore diameter of 15 mmand a thickness of 1.2 mm was made from the optical-disc molding die.

The support base 2 was set in a sputtering system. A phase-changerecorded thin film having a thickness of 170 nm was formed on aninformation-recorded layer 4 of the support base 2. The phase-changerecorded thin film is a recorded thin film composed of several thinfilms of AgPdCu, ZnS SiO₂, AgInSbTe and ZnS.SiO₂.

The support base 2 was set on a spinner as the phase-change recordedfilm was on top of the base. An ultraviolet-hardened resin was droppedonto the phase-change recorded film to form a bonding layer 6.

A sheet 10 made of polycarbonate having a thickness of 100 μm,exhibiting variation in birefringence within ±20 nmpp during onerotation under double-pass measurements was cut into a doughnut-likeinformation-retrieval layer 8 having a major diameter of 119 mm and aminor diameter of 19 mm. The polycarbonate sheet 10 exhibited a low drawratio.

The information-retrieval layer 8 was placed on the bonding layer 6 androtated by the spinner to spread the ultraviolet-hardened resin over thephase-change recorded film to have a specific resin thickness. The sheetinformation-retrieval layer 8 was exposed to a UV light, so that thesheet was stuck on the support base 2, thus finishing an optical disc Ddaccording to the fourth embodiment.

The optical disc Dd was set on known optical-disc initializing equipmentto have the information-recorded layer initialized.

The optical disc Dd was set on the optical-disc birefringence measuringequipment offered by Adomond Science Co. Birefringence was measured withlaser beams having a wavelength of 780 nm incident at 80 to 90 degrees(almost vertical) while the optical disc Dd was rotating. Themeasurements showed two-cycle variation in birefringence during onerotation of the optical disc Dd, which was ±20 nmpp (nano meter peak topeak) under double-pass measurements, like shown in FIG. 3.

Moreover, the optical disc Dd was set on a next-generation optical-discrecorder/player having specifications: a wavelength of 400 nm for theretrieving laser beams and an aperture number of 0.85 for anoptical-pickup objective lens. A modulated signal with the shortest marklength of 0.15 μm was recorded on the optical disc Dd.

The evaluation test with the same optical-disc recorder/player showedtwo-cycle variation in peak-to-peak reproduced output during onerotation of the optical disc Dd, like shown in FIG. 4. In detail, thevariation in reproduced output was 15% obtained by dividing thedifference between the maximum output V_(H) and the minimum output V_(L)by the maximum output V_(H), or (V_(H)−V_(L))/V_(H).

Decreased with the variation in reproduced output, like shown in FIG. 4,were asymmetrical variation in reproduced output and a ratio of theshortest-length bit output to the longest-length bit output. Moreover,almost no errors occurred in decreased signal output. Overall evaluationof the optical disc Dd according to the fourth embodiment resulted inhigh signal quality and stable tracking.

Embodiment 5

An optical disc De according to the fifth embodiment was manufacturedthrough the same procedures as the fourth embodiment, using apolycarbonate sheet having a thickness of 100 μm but exhibiting a drawratio lower than that for the sheet used in the fourth embodiment.

The polycarbonate sheet used in the fifth embodiment exhibited ±10 nmpp(nano meter peak to peak) two-cycle birefringence during one discrotation under measurements with the same birefringence measuringequipment as the fourth embodiment.

The evaluation test with the same next-generation optical-discrecorder/player as the fourth embodiment showed 10% two-cycle variationin reproduced output during one rotation of the optical disc De.

Decreased with the variation in reproduced output were asymmetricalvariation in reproduced output and a ratio of the shortest-length bitoutput to the longest-length bit output. Moreover, almost no errorsoccurred in decreased signal output. Overall evaluation of the opticaldisc De according to the fifth embodiment resulted in high signalquality and stable tracking.

Embodiment 6

An optical disc Df according to the sixth embodiment was manufacturedthrough the same procedures as the fourth embodiment, using an acetatesheet having a thickness of 100 μm but exhibiting a draw ratio lowerthan that for the sheet used in the fifth embodiment.

The acetate sheet used in the sixth embodiment exhibited ±5 nmpp (nanometer peak to peak) two-cycle birefringence during one disc rotationunder measurements with the same birefringence measuring equipment asthe first embodiment.

The evaluation test with the same next-generation optical-discrecorder/player as the fourth embodiment showed 5% two-cycle variationin reproduced output during one rotation of the optical disc Df.

Decreased with the variation in reproduced output were asymmetricalvariation in reproduced output and a ratio of the shortest-length bitoutput to the longest-length bit output. Moreover, almost no errorsoccurred in decreased signal output. Overall evaluation of the opticaldisc Df according to the sixth embodiment resulted in high signalquality and stable tracking.

[Sample 2]

An sample optical disc Dsb was manufactured through the same proceduresas the fourth embodiment, but using a polycarbonate sheet having athickness of 100 μm under drawing in two directions.

The polycarbonate sheet used for this sample exhibited ±45 nmpp (nanometer peak to peak) two-cycle birefringence during one disc rotationunder measurements with the same birefringence measuring equipment asthe fourth embodiment.

The evaluation test with the same next-generation optical-discrecorder/player as the fourth embodiment showed 25% two-cycle variationin reproduced output during one rotation of the sample optical disc Dsb,which was larger than the fourth to the sixth embodiments.

Increased with the variation in reproduced output were asymmetricalvariation in reproduced output and a ratio of the shortest-length bitoutput to the longest-length bit output. Moreover, several errorsoccurred in decreased signal output. Overall evaluation of the sampleoptical disc Dsa resulted in low signal quality and unstable tracking.

As already discussed, it is revealed that the next generation opticaldiscs having the information-recorded layer 4 on the support base 2 andthe transparent information-retrieval layer 8, thinner than the base 2,on the layer 4 suffer low quality in reproduced signals due to variationin reproduced output caused by variation in birefringence on theinformation-retrieval layer 8 during one disc rotation.

This invention solves this problem by suppressing distribution ofbirefringence on the information-retrieval layer 8 can be a specificdirection. In detail, such birefringence is suppressed by drawing in aspecific direction at a specific draw ratio or less in sheet production.

The present invention is not limited to the several embodiments ofoptical discs having the information-recorded layer 4 on the supportbase 2 and the transparent information-retrieval layer 6 on the layer 4via the bonding layer 8. In other words, several modifications areavailable as shown in FIGS. 5A to 5D.

An optical disc D2 shown in FIG. 5A has the information-recorded layer 4formed, not on the support base 2, but between the bonding layer 6 andthe top transparent information-retrieval layer 8.

A double-layered optical disc D3 shown in FIG. 5B has twoinformation-recorded layers 4, the first formed on the support base 2and the second between the bonding layer 6 and the top transparentinformation-retrieval layer 8.

A multiple-layered optical disc D4 shown in FIG. 5C has threeinformation-recorded layers 4, the first formed on the support base 2,the second between the bonding layer 6 and the middle transparentinformation-retrieval layer 8, and the third between a sheet layer 20 onthe middle layer 8 and the top transparent information-retrieval layer8.

Moreover, an optical disc D5 shown in FIG. 5D (like CD with no recordedlayer on a transparent support base) has an information-recorded layer 4of ultraviolet-hardened resin, on the support base 2, formed by 2Pmolding. The support base 2 may be cut out from a 1.2 mm-thick sheet ora 0.6 mm-thick sheet for DVD.

The information-retrieval layer 8 may be of a double-layer structure ofa plastic sheet and a hard-coat layer.

The hard-coat layer may be of thermally hardened resin that allows morethan 70% of light at wavelength λ to pass therethrough,ultraviolet-hardened resin, visible-light-hardened resin,electron-beam-hardened resin, moisture-hardened resin,plural-liquid-mixture-hardened resin, solvent-contained-hardened resin,etc. Such materials for the hard coat layer are preferably at a certainlevel or higher in a pencil-scratch hardness test under JIS standardsK5400 for high wear resistance.

In detail, such materials for the hard-coat layer are preferably at thelevel “H” or higher in the pencil-scratch hardness test under JISstandards K5400 against glass, the hardest material for an objectivelens. The materials under the level “H” could suffer dust when thehard-coat layer is scratched, which results in high error rate.Thickness of the hard-coat layer is preferably 0.001 mm or thicker forhigh impact resistance but 0.01 mm or thinner against warp in theoptical disc for its whole body.

Moreover, the hard-coat layer may be of a thin film of carbon,molybdenum or silicon, or an alloy including each of these metals(including oxide, nitride, sulphide, fluoride or carbide of thesemetals), formed by vacuum deposition. Thickness of the hard-coat layerof a thin film formed by vacuum deposition is preferably 1 nm or thickerfor high impact resistance but 1000 nm or thinner against warp in theoptical disc for its whole body.

The optical disc according to the present invention may be packed in acartridge so that it can be easily set on a player and withstand roughhandling. Moreover, the optical disc according to the present inventionmay be of several sizes, for example, in the range from 20 to 400 mm indiameter. In detail, the diameter may be 32, 41, 51, 60, 65, 80, 88,130, 200, 300 and 356 mm in addition to a standard diameter 120 mm.

The signal format to be recorded on the optical discs is preferably adigitally modulated signal called (d, k) code. The (d, k)-modulatedsignal can be handled as either the fixed-length code or thevariable-length code. The types of modulation suitable for recordingsignals on the optical discs are preferably (2, 10) modulation, (1, 7)modulation and (1, 9) modulation, all for fixed-length code, and also(2, 7) modulation and (1, 7) modulation, both for variable-length code.

Representatives of the (2, 10) modulation for fixed-length code are 8/15modulation (disclosed in Japanese Unexamined Patent Publication No.2000-286709), 8/16 modulation (EFM Plus) and 8/17 modulation (EFM).

One representative of the (1, 7) modulation for fixed-length code is D1,7 modulation (disclosed in Japanese Unexamined Patent Publication No.2000-332613.

One representative of the (1, 9) modulation for fixed-length code is D4,6 modulation (disclosed in U.S. Patent Published Application No.2002-0093751 A1.

One representative of the (1, 7) modulation for variable-length code is17PP modulation (disclosed in Japanese Unexamined Patent Publication No.11-346154/1999.

As disclosed in detail, variation in birefringence during one discrotation is adjusted within ±20 nmpp (nano meter peak to peak) underdouble-pass measurements among birefringence components generated on theinformation-retrieval layer of the optical discs according to thepresent invention. The adjustments for decreasing variation inbirefringence during one disc rotation suppresses variation inreproduced output, thus achieving high reproduction performance.

1. An optical disc comprising: a disc-like support base; and at leastone disc-like information-retrieval layer having an almost same size asthe support base and formed over the support base, theinformation-retrieval layer being thinner than the support base and madeof a sheet cut out from a roll of a sheet material and cut into adisc-like sheet before formed over the support base so that thedisc-like sheets exhibits a two-cycle variation in birefringence within±20 nmpp (nano meter peak to peak) under double-pass measurements duringone disc rotation, wherein the information-retrieval layer at leasthaving a hard-coat layer, and variation in reproduced output of 15% orlower obtained by dividing a difference between maximum and minimumoutputs by the minimum output.
 2. The optical disc according to claim 1,wherein the sheet has a draw ratio adjusted so that the sheet exhibitsvariation in birefringence within ±20 nmpp (nano meter peak to peak)during one rotation under double-pass measurements.
 3. The optical discaccording to claim 1, wherein the sheet is made from a material selectedfrom the group consisting of polycarbonate, polystyrene, amorphouspolyolefine and acetate.
 4. The optical disc according to claim 1further comprising: at least one information-recorded layer; and abonding layer via which the information-retrieval layer and theinformation-recorded layer are stacked each other on the support base.